(1) Field of the Invention
The present invention relates to a position measuring apparatus and a position measuring method for measuring a position of a position measurement target, based on a plurality of images captured from mutually different viewpoints.
(2) Description of the Related Art
A monitoring camera which captures images of a suspicious person, package, vehicle, or the like, and/or a navigation system of a moving or mobile robot require(s) detailed three-dimensional position information of objects present in a range of 360 degrees.
As a typical means for obtaining three-dimensional coordinates, stereoscopic viewing is known which involves detecting corresponding points between images captured by cameras arranged at mutually different viewpoints such that ranges for capturing images are overlapped with each other, and calculating three-dimensional distances between the cameras and the objects based on the amounts of disparity (parallax) between each of pairs of corresponding points.
In addition, recent years have seen emergence of cameras configured to increase a camera's view field to 360 degrees. Examples of such cameras include fisheye cameras using a fisheye lens having a wide imaging angle, and small omnidirectional cameras each composed of (i) a compound mirror which is a combination of a paraboloid and a hyperbolic mirror and (ii) a camera.
Such an omnidirectional stereo camera captures images in a polar coordinate system. Thus, it is impossible to detect corresponding points using a typical stereoscopic viewing technique for handling images captured in an orthogonal coordinate system.
A technique for measuring positions using a small omnidirectional camera is disclosed in Non-patent Literature (NPL) 1 (Ryusuke Sagawa, Yuichiro Kojima, Yasushi Yagi, “Narrow Baseline Omnidirectional Stereo using Disparity Detection Filter”, Meeting on Image Recognition and Understanding (MIRU, 2007), pp. IS-3-31 (2007)). First, an epipolar plane and an epipolar line are calculated using parameters (the curvature of a paraboloidal mirror, the position of an apex, a focal point, etc.) stored inside the camera. Subsequently, corresponding points are calculated by introducing a parallax detection filter to synthesize the parallax of the images smoothed along the epipolar line using windows having various sizes, and positions on omnidirectional images are measured based on the amounts of the parallax.